Topographical organization of subicular neurons projecting to subcortical regions.

Direct projections from the subiculum to the septum, thalamus, and hypothalamus were studied in the rat by the fluorescent retrograde double-labeling technique with Fast blue and Diamidino yellow. The results confirm and extend the previously reported findings. The dorsal subiculum projects primarily to the lateral septum, anterior and midline thalamus, and mammillary complex. The distribution areas of cell bodies of these projection neurons are substantially segregated, depending on their target region, and few single neurons project to two of the target regions by way of axon collaterals. The ventral subiculum projects mainly to the lateral septum, midline thalamus, and ventromedial hypothalamic area. The distribution areas of cell bodies of these projection neurons are considerably overlapped with one another, and a number of single neurons send axon collaterals to two of the lateral septum, midline thalamus, and ventromedial hypothalamic area. It is, thus, indicated that the populations of subicular neurons projecting to each of the subcortical structures examined are more distinctly segregated in the dorsal subiculum than in the ventral subiculum.